1. Field of the Invention
The present invention relates generally to telephone subscriber line voltage and current feed circuitry and more particularly to telephone subscriber loop current limiting for reducing power dissipation in the subscriber line feed circuitry while introducing attenuation/gain in the communication paths to maintain normal telephone equalizer operation.
2. Description of the Prior Art
In standard telephone systems, communications are normally transmitted and received over subscriber lines to and from the telephone subscriber subsets over transmission lines having different loop lengths, as determined by the distance of the calling or called subscriber from the appropriate central or local office. Thus, the telephone signals are subject to varying amounts of attenuation depending upon the line impedance of the line in use, which is of course greater for a "long loop" than for a "short loop", and from a d.c. viewpoint, results in decreased battery feed current in the longer loops.
Various prior art techniques are known to provide compensation for variations in subscriber loop length in telephone operation. U.S. Pat. No. 3,823,273 describes the provision of automatic attenuation and equalization in the transmit and receive paths in accordance with subscriber loop length. U.S. Pat. No. 3,903,378 describes a gain control circuit for controlling amplifier gain in accordance with loop length. U.S. Pat. No. 3,914,560 describes a telephone AGC repeater which automatically adjusts AC signal gain to compensate for loop transmission loss. U.S. Pat. No. 4,056,691 describes a telephone subscriber line circuit for supplying a constant current to the subscriber set substantially independent of loop length.
Telephone local and central offices require circuitry for the provision of battery feed current, supervision, signalling and testing, ringing, loop testing and other voltages and currents required by the subscriber station served thereby in providing the requisite subscriber line interface. The battery power (which is a term of art referring collectively to the voltage and current supplied for all of the above functions) and the heat which must be dissipated by the line circuit are important design factors in the interface circuitry, especially when large scale integrated circuits (LSI) are used. It is desirable to minimize the size, cost, battery power and heat dissipated in the subscriber line circuitry.
A telephone subset equalizer, as is well known, includes means for equalizing the gain levels in both the transmitting and receive circuitry of the subset as a function of the subset distance from the central or local office. This is accomplished by circuitry within the equalizer for automatically increasing the gain at the subset when a low d.c. loop current is sensed at the subset by the equalizer. The operating battery voltage supplied from the central office is typically -48 volts and the d.c. loop current which varies with loop length and associated line resistance varies typically between 80 milliamps and 20 milliamps. The equalizer opertion is well known, and consequently is not described in detail herein; however, reference is made to Transmission Systems for Communication, Chapter 3, pgs. 50-55 by Bell Telephone Laboratories, 3rd ed. 1964 for a more detailed description of equalizer operation, including schematics of the equalizer circuitry for a standard Western Electric 500-D type telephone, and including curves of the relative response in decibels of the telephone set for various loop lengths, with and without the described equalizer.
Absent regulation, the telephone sound level would increase inversely with the distance of the subscriber set from the central office due to equalizer boost. To eliminate this undesired effect, prior art equalizers limit the sound level after sensing the loop current by controlling the gain of the equalizer amplifiers in the transmit and receive circuits to compensate for the equalizer boost. Such a technique is described by the aforementioned U.S. Pat. No. 3,903,378.